Power transmission

ABSTRACT

A hydraulic pump or motor of the axial piston type having a cylinder barrel rotating against a fixed valve plate perpendicular to the shaft and free to find its own seat against the valve plate has an improved casing which restricts circulation of liquid tending to be carried around with the rotating cylinder barrel. This reduces the magnitude of the fluid pressure forces exerted on the cylinder barrel by the circulating fluid and which tend to tip the cylinder barrel away from full contact with the valve plate which consequently increases the maximum speed at which the unit can operate.

United States Patent Chondzinski 1451 Sept. 26, 1972 1 POWER TRANSMISSION [72] Inventor: Edward J. Chondzinski, Detroit,

Mich.

[73] Assignee: Sperry Rand Corporation [22] Filed: Dec.28, 1970 1 {211 App]. No.: 101,647

[56] References Cited UNITED STATES PATENTS 10/1939 Snader et al...,. ..91/499 5/1968 Gantzer .....91 499 8/1971 Oram ..9l/499 FOREIGN PATENTS OR APPLlCATIONS 956,824 10/1949 France ..9l/499 Primary Examiner-Paul E. Maslousky Attorney-Van Meter and George [5 7] ABSTRACT A hydraulic pump or motor of the axial piston type having a cylinder barrel rotating against a fixed valve plate perpendicular to the shaft and free to find its own seat against the valve plate has an improved casing which restricts circulation of liquid tending to be carried around with the rotating cylinder barrel. This reduces the-magnitude of the fluid pressure forces exerted on the cylinder barrel by the circulating fluid and which tend to tip the cylinder barrel away from full contact with the valve plate which consequently increases the maximum speed at which the unit can operate.

5 Claims, 4 Drawing Figures POWER TRANSMISSION In axial piston pump or motor units having a revolving cylinder barrel mounted to find its own seat against a stationary valve plate, centrifugal moments are developed by reason of the differing projection of the pistons out of their respective cylinder bores which moments tend to tip the cylinder barrel away from the valve plate. These moments have been counteracted heretofore by resilient means or fluid pressure means acting to hold the cylinder barrel in contactwith the valve plate. The magnitude of such hold-down forces which will be required for any given maximum speed of the unit, are readily calculable. It has been found in practice, however, with such units designed for, high speeds, that the full theoretical speed cannot be achieved except by the provision of hold-down means far stronger than the calculations require, and this results in excessive frictional drag and wear between the cylinder barrel and valve plate.

The present invention is based upon the discovery that the body of liquid withinthe casing of the unit, which is carried around by frictional drag with the cylinder barrel and the projecting pistons, exerts a laterally unbalanced pressure force acting to tip the cylinder barrel away from the valve plate and that this force can be eliminated by properly shaping the interior surface of the casing of the unit to minimize the pressure forces which this circulating fluid exerts on the cylinder barrel.

This is achieved in a hydraulic pump or motor of the swash plate type which has a revolving cylinder barrel carrying axial pistons which transmit the driving torque as cantilevers and having a drive shaft and a valve plate perpendicular thereto with the cylinder barrel connected to the shaft with freedom to find its seat against the valve plate by the provision of that improvement which includes a casing having a non-concentric inner contour surrounding the cylinder barrel which is shaped to control the circulation of fluid carried around by the drag of the cylinder barrel for preventing the generation of laterally unbalanced pressure forces on the barrel.

. IN THE DRAWING FIG. 1 is a longitudinal cross section of a piston pump or motor unit incorporating a preferred form of the present invention.

FIG. 2 is a fragmentary sectional view taken along line 2--2 of FIG. 1.

FIG. 3 is a diagrammatic view illustrating some of the forces acting on the cylinder barrel.

FIG. 4 is a diagrammatic view illustrating the fluid flows within the casing.

Referring to FIG. 1, a typical axial piston pump or motor unit has a casing and a valve plate 12 within which a drive shaft 14 is journalled on bearings 16 and 18. Inlet and outlet connections 20 are provided in the valve plate 12 which lead to the usual arcuate commutating ports in the flat surface 22 of the valve plate 12. The casing 10 includes a fixed swash plate 24 around which piston shoes 26 slide, carrying the pistons 28 which project from their respective cylinder bores 30 in a revolving cylinder barrel 32. The barrel 32 is connected to the shaft 14 by a spline 34 which allows a slight amount of universal action which permits the barrel 32 to find its own seat against the flat valving surface 22 of the valve plate 12. A spring 36 acts to hold the cylinder barrel in contact with the valve plate as a force additional to the hydraulic force generated within the shouldered ends of the cylinder bores 30.

The foregoing description is typical of axial piston units as heretofore designed. Referring now to FIG. 3 which illustrates diagrammatically, in exaggerated form, the couples acting upon the cylinder barrel which either assist or oppose its tendency to find a seat flat against the valve plate in its freedom to move about the center A of the spline 34. Thus, the moment generated by centrifugal force acting on the pistons with their varying degrees of projection is represented by the couple B. This couple tends to lift the cylinder barrel off from the valve plate as it may pivot about the center A.

' Counteracting this tendency is the couple C generated by the spring 36. These couples may be readily calculated and the force of spring 36 so chosen as to hold the cylinder barrel in contact with the valve plate up to any desired maximum speed. It has been found in practice, however, that particularly at the higher speeds a much stronger spring than the one calculated to be necessary must be used if the cylinder barrel is to be maintained from tipping. It is, of course, undesirable to use high hold-down pressures either from the spring or from the hydraulic forces within the cylinders 30 if undesirable friction and wear are to be avoided at the valving surfaces.

In pump or motor units as heretofore constructed, when operated with a flooded casing, the .high speed rotation of the cylinder barrel and pistons causes a rotary circulation of the body of fluid within the casing 10. This body of fluid, as it passes through the area represented by the top portion of FIG. 1, occupies an area whose axial extent is represented by the dimension D, whereas, when it passes through the bottom portion of FIG. 1, it has a much smaller area represented by the axial dimension E. This results in a substantial difference in velocity between these two areas. The total energy being substantially constant throughout the body of circulating liquid, it follows that the dynamic or velocity pressure of the fluid passing through the bottom area of FIG. 1 is very high and that passing through the top area is relatively low. The static pressures, however, are just the opposite and it is the static pressures which exert forces upon the cylinder barrel 32. These forces are represented by the dotted arrow F in FIG. 3 as they exist in conventional units. Thus, the force F acting around the center A tends to tip the cylinder barrel away from the valve plate as shown in FIG. 3.

To eliminate or greatly reduce this force, the present invention provides an interior configuration which controls the flow of the circulating liquid. Preferably, these include a pair of baffles 38 on the interior surface of the casing 10 which act to block a major portion, at least, of the liquid which tends to circulate around the interior of the casing by being dragged along with the rotary cylinder barrel and its projecting pistons. These baffles are preferably two in number and are located circumferentially of the device at the points where the pistons are in the mid point of their stroke, that is to say, on the mid plane of the device, as illustrated in FIG. 1. These baffles may take any particular form, either being cast integrally with the interior surface of the casing 10 or inserted as separate elements 38, such as illustrated in FIGS. 1 and 2. it is preferred to keep the clearance between the cylinder barrel 32 and the baffles 38 as small as reasonably possible and to thus interrupt or at least drastically restrict any circulation of the fluid contained within the casing 10. Thus, the force G, as seen in FIG. 3, which is exerted by the static component of the total fluid pressure, may be made very nearly equal to the force F exerted on the opposite side of the cylinder barrel 32. At the same time, the very limited arc over which the baffles extend avoids the creation of significant fluid shear forces at the close clearance with the barrel.

Referring now to FIG. 4, where the rotary circulating flows are diagrammed, the velocity of the fluid which circulates with the cylinder barrel in a-conventional device is plotted at H and I. On the side of the cylinder barrel, represented at the top of FIG. 1, the velocity across the cross-section of fluid is represented by the area H which has only a low maximum velocity. n the other hand, at the area I, the maximum velocity is very high. These are representative of the flows which occur in a conventional unit of the type under consideration. By the provision of the baffles 38, the continuous rotary circulation of any substantial quantity of fluid is prevented and the major portion of such circulation which takes place is a re-entrant one as illustrated by the dotted arrows 40. Thus, the magnitude of the static pressure force represented by the arrows F and G in FIG. 3, are brought nearly into equilibrium and the tendency which would otherwise be exerted by the circulating fluid to tip the cylinder barrel off from the valve plate is eliminated or greatly reduced. This enables units designed for high speeds to operate at their full rated speed without excessive wear at the valve plate surface.

I claim:

1. In a hydraulic pump or motor of the type having a swash plate, a revolving cylinder barrel carrying axial pistons which transmit the driving torque as cantilevers and having a drive shaft and a valve plate perpendicular thereto, with the cylinder barrel connected to the shaft with freedom to find its seat against the valve plate, that improvement which includes a casing having a nonconcentric inner contour surrounding the cylinder barrel which is shaped to control the circulation of fluid carried around by the drag of the cylinder barrel for preventing the generation of laterally unbalanced pressure forces on the cylinder barrel.

2. A pump or motor unit as defined in claim 1 wherein the shaped inner contour includes a plurality of flow restricting means for liquid which would otherwise tend to circulate with the cylinder barrel.

3. In a hydraulic pump or motor of the type having a swash plate, a revolving cylinder barrel carrying axial pistons which transmit the driving torque as cantilevers and having a drive shaft and a valve plate perpendicular thereto, with the cylinder barrel connected to the shaft with freedom to find its seat adjacent the valve plate, that improvement which includes a casing having an internal configuration adjacent the cylinder barrel which is baffled to interrupt the circulation of fluid around the cylinder barrel between the valve plate and the swash plate.

4. A pump or motor as defined in claim 3 wherein two baffles are provided which project inwardly to occupy most of the clearance between the cylinder barrel and the casing.

5. A pump or motor unit as defined in claim 4 wherein the baffles are located in a plane occupied by the pistons at the mid point of their stroke. 

1. In a hydraulic pump or motor of the type having a swash plate, a revolving cylinder barrel carrying axial pistons which transmit the driving torque as cantilevers and having a drive shaft and a valve plate perpendicular thereto, with the cylinder barrel connected to the shaft with freedom to find its seat against the valve plate, that improvement which includes a casing having a non-concentric inner contour surrounding the cylinder barrel which is shaped to control the circulation of fluid carried around by the drag of the cylinder barrel for preventing the generation of laterally unbalanced pressure forces on the cylinder barrel.
 2. A pump or motor unit as defined in claim 1 wherein the shaped inner contour includes a plurality of flow restricting means for liquid which would otherwise tend to circulate with the cylinder barrel.
 3. In a hydraulic pump or motor of the type having a swash plate, a revolving cylinder barrel carrying axial pistons which transmit the driving torque as cantilevers and having a drive shaft and a valve plate perpendicular thereto, with the cylinder barrel connected to the shaft with freedom to find its seat adjacent the valve plate, that improvement which includes a casing having an internal configuration adjacent the cylinder barrel which is baffled to interrupt the circulation of fluid around the cylinder barrel between the valve plate and the swash plate.
 4. A pump or motor as defined in claim 3 wherein two baffles are provided which project inwardly to occupy most of the clearance between the cylinder barrel and the casing.
 5. A pump or motor unit as defined in claim 4 wherein the baffles are located in a plane occupied by the pistons at the mid point of their stroke. 